A pulse oximeter is a sensing device for non-invasive measurement of a person's arterial-blood oxygen saturation level. The measurement is done by measuring absorbances of light beams having pre-determined wavelengths after the light beams travel through or are reflected by a pre-determined part of the person's body. Hereinafter, a light beam directed to the pre-determined part of the body for absorbance measurement is referred to as a probe light beam. For a transmissive oximeter, the probe light beams are usually directed to one side of a thin section of the body such as a finger, a palm or an earlobe, and light sensors are used to measure intensities of the light beams that exit this thin section from the opposite side. For a reflective oximeter, the probe light beams may be directed to the skin of a foot, forehead or chest, and photosensors are used to detect the reflected light beams. Typically, two pre-determined wavelengths of 660 nm and 940 nm are used for the probe light beams of the pulse oximeter. The probe light beam having a wavelength of 660 nm is visible and is red light while it is infrared (IR) light for the light beam of 940 nm in wavelength. For background details on a pulse oximeter and its operational principle, refer to J. S. GRAVENSTEIN, Gas monitoring and pulse oximetry, Butterworth-Heinemann Limited, 1990, the disclosure of which is incorporated by reference herein.
It is sometimes required to calibrate a pulse oximeter, for example, to check its accuracy before doing measurement for a person or a patient. A calibrator for calibrating the pulse oximeter is used. One requirement of the calibrator is to emulate absorbance characteristics of the probe light beams propagated in the pre-determined part of human body by replicating absorption behavior of each of the probe light beams when the probe light beams encounters human tissues, blood, bones, etc. in the aforementioned part of body. Another requirement is to emulate change of absorbance caused by pulsing arterial blood and experienced by each of the probe light beams when the probe light beams propagate in the pre-determined part of body.
In U.S. Pat. No. 5,166,517, a manually operable calibrator for checking the accuracy of a pulse oximeter has a layered structure, where the pulse oximeter that can be calibrated by this calibrator is a transmissive oximeter. The calibrator comprises a specially prepared liquid and a resiliently flexible displaceable member adjacent to the liquid. The liquid is prepared for emulating absorption behavior of probe light beams traveled in the pre-determined body part. By manually pumping the liquid into and out of the displaceable member in a rhythmic manner, absorbance change due to pulsing arterial blood is emulated. One disadvantage of the calibrator is that its layered structure increases manufacturing costs but lowers the calibrator reliability due to involvement of a plurality of components. Another disadvantage is that liquid is involved, increasing difficulty in handling and in storage. Liquid media are also used in other calibrators, such as the one disclosed in China Patent Application Publication No. 1,864,629.
China Patent Application Publication No. 1,836,632, addressing an issue that existing calibrators in the market easily lead to the over-driven problem during calibration, discloses a light scattering medium in an attempt to solve this problem. The light scattering medium is made of epoxy resin and is incorporated with a selected, preferred volume percentage of scattering materials. When this light scattering medium is used in an oximeter calibrator, the resultant calibrator will have a layered structure with a number of components. The involvement of a number of components increases manufacturing costs and reduces the calibrator reliability.
There is a need in the art for a calibrator having a simple structure realized by minimal components without involving any liquid medium. It is desirable if the calibrator can be used for calibrating both transmissive oximeters and reflective oximeters.